Frogs with skin cysts or shortened or missing legs make up only 2 percent of the amphibians collected during a 10-year study. The results suggest that malformed frogs and toads are much rarer than scientists first reported in the mid-1990s.

Scientists gathered more than 62,000 frogs and toads at 152 U.S. refuges from 2000 to 2009. Among the sites, there were hotspots in California, Alaska and the Mississippi River Valley and other places where malformations were more common. But frogs and toads in these areas often had abnormal features for a year to a few years and then the animals would recover, the researchers report November 18 in PLOS ONE. Only 12 frogs had extra legs.

While the study does not pinpoint specific causes for the animals' abnormalities, the findings do suggest that the malformations are not a result of random errors during development or changes in climate, such as extra rainfall or drought. The results should also help researchers understand if malformation hotspots have an influence on the overall decline in U.S. and global amphibian populations, the scientists say.